Environ. Sci. Technol. 1998, 32, 807-812
Dynamics of Zebra Mussel Oxygen Demand in Seneca River, New York† STEVEN W. EFFLER* AND STEPHEN R. BOONE Upstate Freshwater Institute, P.O. Box 506, Syracuse, New York 13214 CLIFFORD SIEGFRIED Biological Survey of the New York State Museum, Albany, New York 12230 STEVEN L. ASHBY USAE Waterway Experimental Station, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180-6199
The magnitude and dynamics of oxygen depletion from zebra mussel metabolism for a severely infested 1.4 km section of the Seneca River, NY, is documented for a 3-month period of 1994, based on hourly measurements of dissolved oxygen (DO) concentrations at an upstream site and a downstream site and mass balance calculations. The average areal zebra mussel oxygen demand (ZOD) in the study section during a 30-day low-flow period (population density ∼40 000 individuals m-2) was 39.4 g m-2 d-1. The average value of ZOD for the same 30-day period, normalized for zebra mussel biomass mass (42.4 mg of O2 (g of zebra mussel)-1 d-1), compares favorably to recently reported oxygen consumption rates from laboratory experiments, supporting its application elsewhere to accommodate the effects on oxygen resources of invaded systems. Values of ZOD varied greatly during the study reflecting responses of the invader to changes in ambient conditions, including flow and a major decrease in the population.
Introduction The zebra mussel, Dreissena polymorpha (Pallas), has been one of the most successful invaders of the aquatic ecosystems of North America (1). This filter-feeding bivalve has spread through much of the eastern United States and parts of the Midwest (2) since introduction to the Great Lakes in the mid1980s (3). Dense populations of zebra mussels can have major impacts on water quality, associated with various aspects of their metabolism (4), including increased clarity (4-6), mobilization of nutrients (7-10), and oxygen depletion (4, 11). The additional oxygen sink represented by respiration demands of zebra mussels is of particular concern in lotic environments in developed areas, where oxygen resources may be limited (4, 11). Many rivers in developed areas have little or no assimilative capacity to lose for oxygen-demanding wastes. Effler and Siegfried (11) attributed the major decrease in summertime dissolved oxygen (DO) concentrations in † Contribution No. 177 of the Upstate Freshwater Institute; Contribution No. 776 of the New York State Biological Survey. * Author to who all correspondence should be addressed. Telephone: 315-466-1309; fax: 315-466-8289; e-mail: mgperkins@ aol.com.
S0013-936X(97)00838-9 CCC: $15.00 Published on Web 02/03/1998
1998 American Chemical Society
portions of the Seneca River, NY, in 1993 to a severe infestation of zebra mussels (∼50 000 individuals m-2). An areal respiration rate (or flux) for this population of about 34 g m-2 d-1 was estimated for a 1.4 km section of the river (11) based on a single quantitative biological survey of this sessile organism and application of respiration relationships reviewed by Schneider (12). This approximately matched the areal oxygen sink calculated independently from DO budget calculations (44 g m-2 d-1) based on measurements of DO depletion across the same river section on three occasions in a single month (11). The areal oxygen consumption rate, described as zebra mussel oxygen demand [ZOD (4)], depends on the area of colonization and the density, size structure, and metabolic state of the mussel population within the area (4, 11-13). The oxygen sink process(es) associated with zebra mussels is properly accommodated within the frameworks of mathematical mass balance models of DO by the value of ZOD (4). The magnitude of ZOD should be expected to vary not only as a result of changes in the zebra mussel population but also in response to changes in ambient conditions and related stresses to the bivalve; e.g., variations in the quantity and quality of food (14, 15) and nonfood particles (16), flow (8), and temperature (13). Here we document the magnitude and dynamics of oxygen depletion from zebra mussel metabolism and related estimates of net areal rates of depletion for an infested section of the Seneca River for a 3-mo-interval of 1994, based on mass balance calculations from frequent measurements of DO at an upstream site and a downstream site (bounding sites). The observed dynamics are analyzed within the context of variations in ambient conditions and a major change in mussel biomass (8) during the study. Estimates of biomass-specific DO depletion rates are compared to recently reported oxygen consumption rates determined in laboratory experiments (13).
Study Site General. The Seneca River is an alkaline hardwater system that drains ∼9000 km2 of the Finger Lake region of New York to Lake Ontario. The site for this study is a 1.4 km section of the river, extending from 1.3 km downstream of Cross Lake (∼72 km upstream of Lake Ontario), known as the State Ditch Cut (Cut; Figure 1). The Cut (depth of 3.5-4.0 m) is one of a number of anthropogenic alterations of the river (e.g., channelization, dams, and locks) to support navigation and hydroelectric power generation (17). This navigation channel was cut from bedrock; the walls are nearly vertical, and the bottom substrate is “cobble” size rock. No pointsource waste inputs enter the river over the length of the study section. Cross Lake is a rapidly flushing (∼50 times yr-1) dimictic hypereutrophic system (18). Filamentous cyanobacteria generally dominate the phytoplankton of Cross Lake during late summer (19). Certain cyanobacteria are apparently not utilized as food by zebra mussels (14). Zebra mussels occupied all available cobble substrate in the Cut in late summer of 1993; the size structure of the population indicated the vast majority of the infestation developed in 1992 and 1993 (11). The average population density in the Cut in 1993 was ∼50 000 individuals m-2 (11), approaching maxima reported for the Great Lakes (20, 21). The average areal biomass (dry tissue weight) of zebra mussels was 712 g m-2 (11). The particularly dense populations found in the Cut have been attributed to the abundance of both food (e.g., phytoplankton from Cross Lake) and rock substrate (11). VOL. 32, NO. 6, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
807
FIGURE 1. Seneca River, NY, study reach (Cut). Insets of the monitoring sites within the Cut, and the Seneca River within the Oswego Basin and New York State (modified from Effler et al. (8)). River flow (as measured 17 km downstream; Baldwinsville, Figure 1) varied substantially during the study period (Figure 2a). Flow (Q) was initially relatively high, but by mid-July Q < 30 m3 s-1 and remained low until the onset of a major runoff event in mid-August (Figure 2a). The runoff event lasted until early September, followed by the second lowflow interval of the study; flow increased again in late September (Figure 2a). The minimum average flow for a 30-day period, with a return frequency of 1 in 10 yr, for this portion of the Seneca River is 17.6 m3 s-1 (17). Near-bottom current velocities increased from about 0.12 m s-1 in late July and early August to 0.5 m s-1 during the late August runoff event (8). The dynamics of the concentrations of suspended solids (Figure 2b) and chlorophyll (Figure 2c) were in part linked to variations in flow; e.g., the contribution of the inorganic component to total suspended solids was greater during the three high flow intervals (e.g., sediment resuspension), and the concentrations of the volatile component and chlorophyll were generally lower (e.g., dilution of lake concentrations). The disproportionate decrease in chlorophyll during the late August runoff event (Figure 2b,c) indicates that the food value of organic particles during this interval may have diminished (15). These aggregate measures reflect high concentrations of potential food entering the site, particularly during low flow periods (Figure 2b,c). By comparison, the dynamics of temperature (Figure 2d) were more gradual, typical of the seasonality experienced in this region. Temperature ranged from 17 to 26 °C over the study. Temperature remained near its maximum from early July to early August and decreased progressively thereafter (Figure 2d).
FIGURE 2. Dynamics of selected environmental conditions in the Seneca River, NY, at the upstream boundary of the Cut, JulySeptember, 1994: (a) flow, (b) suspended solids, (c) total chlorophyll, and (d) temperature (modified from Effler et al. (8)). Ambient Conditions and Zebra Mussel Dynamics: Summer of 1994. The dynamics of selected environmental conditions at the upstream boundary of the study site and of the zebra mussel population within the site during the summer of 1994 were described and supporting methodology was specified by Effler et al. (8). Features of these conditions are reviewed here (Figure 2) to support this evaluation of the magnitude and dynamics of oxygen depletion associated with this population. 808
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 6, 1998
Zebra mussel population data for the study period are based on the results of monthly quantitative surveys (multiple stations and replicates within the study site; 11). The zebra mussel population was nearly uniform for the portion of the study before the August runoff event; the average density and biomass (dry tissue weight) in the Cut for this interval were about 40 000 individuals m-2 and 930 g m-2, respectively (8). This population decreased dramatically during the event; e.g., to about 18 000 individuals m-2 and 240 g m-2 , respectively (8). Approximately 38% of the biomass before the abrupt decrease was associated with individuals of shell length of 10-20 mm as compared to 15% after (e.g., shift toward smaller size classes following the event; 8). Analysis of the time structure of nutrient flux from the mussel population indicated the reduction in biomass within the Cut occurred rather abruptly in late August, apparently in
FIGURE 3. Time series of hourly dissolved oxygen (DO) measurements at a depth of 1 m, at the upstream boundary of the Cut, for the July-September interval of 1994. Inset of DO concentrations measured on July 22, 1994. response to the increased velocity of flow during the runoff event (8). By the time of this runoff event, the colonies of zebra mussels were several centimeters thick (i.e., numerous layers of individuals). The early colonizers at the interface with the substrate were subject to mortality and detachment at the modest increase in current velocity (8). Other factors identified (8) as potentially contributing to the abrupt loss of zebra mussel biomass from the site were the high temperatures (13) of July, low DO concentrations (presented subsequently), increased concentrations of inorganic suspended solids, and reductions in edible food particles during the August runoff event. The zebra mussel population of the Cut was estimated to be capable of filtering about 65% of the river flow before the August runoff event; the filtering capability declined to 3 times greater during the high flows of early July than determined during the low flows at the end of the month (Figure 5d). Increases in turbulence have been reported to cause increased respiration in freshwater bivalves (16). However, greater impact is to be expected by the combination of increased turbulence and higher concentrations of nonfood particles (27; see Figure 2b,c). These conditions are observed to be coupled in many systems, as increased turbulence promotes sediment resuspension (28, 29). As concentrations of food particles decrease and nonfood particles increase, this nonselective filter-feeder expends more energy to acquire the necessary food and in psuedofeces production (12, 27). It is likely that the combined effects of turbulence and nonfood particles were primarily responsible for the structure of the dynamics of net areal DO depletion before late August. No clear manifestations of the effects of temperature were observed over this interval. For example, despite increases in temperature in early July (Figure 2d) reported to cause increased respiration in laboratory studies [e.g., ∼ 50% (13)], the areal DO depletion continued to decrease (Figure 5d), largely tracking the pattern of flow. Oxygen sink processes other than those associated with the zebra mussel population (e.g., oxidation of carbonaceous and nitrogenous biochemical oxygen demand in the overlying water column and phytoplankton respiration) as well as source processes (e.g., photosynthesis and reaeration) of course operated in the Cut during the study. However, the available information indicates that the magnitudes of these other processes and their net effect were small relative to the oxygen consumption associated with the zebra mussel population over the time step of 1 day. Data from downstream of Cross Lake and at Baldwinsville (Figure 1) before the zebra mussel invasion (e.g., refs 17 and 19) suggest that the source and sink processes were approximately in balance over this 17 km reach of the river (ie., DO concentrations uniform). Application of kinetic relationships developed specifically for the river before the invasion (17) for conditions encountered in the Cut during the 1994 study period support the position that the magnitudes of these processes were small relative to consumption by the zebra mussels. The largest effect of any of these process during this study was associated with reaeration on those days that substantial undersaturation with respect to DO occurred at the site. We estimate a maximum net oxygen input from this process (17), during the maximum DO deficit intervals of July and August (e.g., 2-3 mg L-1, see Figure 5c), to be
